1. Field of the Invention
The present invention relates to the field of microelectronics, and more specifically to a bipolar transistor and to its implementation.
2. Discussion of the Related Art
Bipolar transistors used as integrated circuit components especially those integrated with MOS transistors in integrated circuits of Bi-CMOS type will more specifically be considered herein.
Integrated circuits with increasingly high performance are developed. To achieve this, all components must be optimized. Bipolar transistors are used because of their dynamic performance which enable their use in the field of very high frequencies, greater than 50 GHz.
A conventional NPN-type transistor is schematized in FIG. 1. It comprises an N-type doped single-crystal silicon collector region 1, a P-type doped single-crystal silicon base region 2, and an N-type doped single-crystal silicon emitter region 3. The collector is formed in a silicon substrate, the base region is formed above the collector region. The emitter region is formed inside of the base region, for example, by diffusion of dopant atoms. A metal 4 is deposited on emitter region 3 to ensure an electric contact on this region. The operation of this bipolar transistor is the following. A control current Ib is injected between the base region and the emitter region and a collector current Ic usable in electronic circuits results therefrom, according to the biasing conditions of the collector region. Collector current Ic is a desired current and base current Ib is a parasitic current. Ratio Ic/Ib, which is the current gain of the bipolar transistor, is a figure of merit that those skilled in the art attempt to increase to obtain values greater than 60. A very large number of technological parameters modify the bipolar transistor gain. If metal 4 is close to the base-emitter junction, a significant base current is created by recombining the minority carriers located close to this junction. Thus, if base current Ib is desired to be decreased, it is necessary to increase the distance between metal 4 and base region 2. In other words, it is necessary to have an emitter with a thickness greater than several times the diffusion length of the minority carriers present in this emitter region. Thus, conventional bipolar transistors have an emitter thickness greater than 800 nm. The presence of metal is not the only parameter with an influence on the gain. The parameters which modify the current gain of bipolar transistors are in a greater number, sometimes poorly understood and often poorly controlled. But it has been observed that an increase in the emitter thickness would always result in a better injection efficiency (increase of Ic) and in a smaller base current Ib.
Bipolar transistors are often used for their dynamic performance. For purely geometric reasons, the structure shown in FIG. 1 is a low-performance structure because of the capacitance present between the base and emitter regions. This capacitance is proportional to the surface area of the junction between the base and the emitter. Since the emitter extends deeply into the base, the contribution of the emitter periphery to the total capacitance between the base and emitter regions is significant while this region, distant from the base/collector junction, plays a reduced part in the bipolar transistor currents. To increase the dynamic performance of the bipolar transistor, bipolar transistors with a polysilicon emitter according to FIG. 2 have been formed.
The transistor of FIG. 2 comprises an N-type doped single-crystal silicon collector region 10 on which is formed a single-crystal silicon base region 20 in and on which is formed an emitter structure 30-35-40. Emitter 35 is made of N-type doped polysilicon and is prolonged, in base region 20, by an N-type diffused region 30. A metal 40 rests on emitter 35. The thickness of single-crystal silicon portion 30 of the emitter is approximately 100 nm and the thickness of polysilicon portion 35 of the emitter is approximately 600 nm. Portion 30 of the emitter has a preponderating contribution for the stray capacitance between the emitter and the base region of the bipolar transistor. The small depth of portion 30 results in that this stray capacitance is greatly reduced with respect to the case of FIG. 1, although the general emitter thickness is similar for the cases of FIGS. 1 and 2.